1. Field of the Invention
This invention relates generally to a method for optimizing a cathode fill strategy for a fuel cell stack at system start-up and, more particularly, to a method for providing the proper amount of cathode air to the cathode side of a fuel cell stack at system start-up that employs a cathode model of the hydrogen concentration on the cathode side of the stack.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte there-between. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between the two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The gas permeation rate for state of the art membranes in PEM fuel cells is relatively low compared to the current produced by the fuel cell for power generation. When the fuel cell system is shut down, the gas permeation continues through the membrane until the gas component partial pressures have equalized on both sides of the membrane. The diffusivity of hydrogen through the membrane from the anode to the cathode is approximately three times the rate of nitrogen from the cathode to the anode. Higher hydrogen diffusion rates equate to a rapid equalization of hydrogen partial pressures compared to a relatively slow equalization of nitrogen partial pressure. The difference in gas diffusivities causes the anode sub-system absolute pressure to drop until the cathode hydrogen partial pressure reaches the anode hydrogen partial pressure. Typically, the anode side of the fuel cell stack is operated at a high hydrogen concentration, such as greater than 60%, and large volumes of hydrogen rich gas exist outside of the anode of the stack. As the anode absolute pressure drops, more hydrogen is drawn out of the anode sub-system into the anode flow field of the stack.
The net result of the hydrogen partial pressure equalization after system shut-down is an increased concentration of hydrogen in the cathode side of the fuel cell stack, at least for some period of time after shut-down. At system start-up, the compressor is started, but the concentration of hydrogen exiting the fuel cell stack from the cathode must be limited to not violate emission requirements. Thus, as the cathode of the fuel cell is filled with fresh air, the hydrogen rich gas leaving the cathode side of the stack must be diluted. To meet start-time and noise requirements, there is a need to optimize the fill of the stack cathode. Because the cathode flow is limited by the power available to the compressor, the fill method must be robust to changes in total compressor flow rate.
Known fuel cell systems typically employ a by-pass valve that allows cathode air to by-pass the fuel cell stack and be directed from the compressor directly to the system outlet. Start-up strategies may employ mechanisms to open the by-pass valve so a considerable amount of air does not go through the cathode of the fuel cell stack and is available at the stack output to dilute the hydrogen that may be forced through the cathode side of the stack. Typically these start-up strategies have been overly conservative to not exceed the desired hydrogen emissions concentration ant any point during the start-ups. Because the actual system start-up must wait for the hydrogen emissions to be diluted before the stack is started, these conservative start-up strategies have increased the time that the system can be started from ignition.